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Friday, 8 March 2013

In a Warming World, Storms May Be Fewer but Stronger (Part 2)

What exactly does it mean for storms to get “stronger”? Does it mean faster winds? A larger wind field? Lower pressure at the center? More rain and snowfall? Higher storm surges?

“You have to remember that storms aren’t one-dimensional,” says Del Genio. “There are many types of storms, and sorting out how aspects of each type respond to warming is where the science really gets interesting.”

As Sandy was moving up the U.S. East Coast, unusually warm ocean
temperatures allowed the storm to stay strong after it left tropical
waters. (Map by Robert Simmon, using data from the NOAA Earth System Research Laboratory.)

Rising sea levels exacerbated Sandy’s storm surge, for example, a
direct link between global warming and storm damage. And abnormally high
sea surface temperatures in the Atlantic probably intensified the
storm. But pinning all of Sandy’s fury—its hybrid nature, the scale of
its winds, its unusual track—on global warming is premature, says
Shepherd, the current president of the American Meteorological Society.

Weather forecasters use terms like snowstorms, derechos,
hailstorms, rainstorms, blizzards, low-pressure systems, lightning
storms, hurricanes, typhoons, nor‘easters, and twisters. Research
meteorologists and climatologists have a simpler way of dividing up the
world’s storms: thunderstorms, tropical cyclones, and extra-tropical
cyclones. All are atmospheric disturbances that redistribute heat and
produce some combination of clouds, precipitation, and wind.

Thunderstorms are the smallest type, and they are often part of the
larger storm systems (tropical and extra-tropical cyclones). All storms
require moisture, energy, and certain wind conditions to develop, but
the combination of ingredients varies depending on the type of storm and
local meteorological conditions.

For example, thunderstorms form when a trigger—a cold front,
converging near-surface winds, or rugged topography—destabilizes a mass
of warm, humid air and causes it to rise. The air expands and cools as
it ascends, increasing the humidity until the water vapor condenses into
liquid droplets or ice crystals in precipitation-making clouds. The
process of converting water vapor into liquid water or ice releases
latent heat into the atmosphere. (If this doesn’t make sense, remember
that the reverse—turning liquid water into water vapor by boiling
it—requires heat).

Storms feed off of latent heat, which is why scientists think
global warming is strengthening storms. Extra heat in the atmosphere or
ocean nourishes storms; the more heat energy that goes in, the more
vigorously a weather system can churn.

Thunderstorms derive their energy from the heat released by the
condensation of water vapor. This “latent heat” energy drives
thunderstorm clouds high into the atmosphere. Thunderstorms dissipate
when the cold downdraft created by falling rain drops stifles rising
warm air. (Image adapted from NOAA National Weather Service Life Cycle of a Thunderstorm.)

Already, there is evidence that the winds of some storms may be
changing. A study based on more than two decades of satellite altimeter
data (measuring sea surface height) showed that hurricanes intensify
significantly faster now than they did 25 years ago. Specifically,
researchers found that storms attain Category 3 wind speeds nearly nine
hours faster than they did in the 1980s. Another satellite-based study
found that global wind speeds had increased by an average of 5 percent
over the past two decades.

There is also evidence that extra water vapor in the atmosphere is
making storms wetter. During the past 25 years, satellites have measured
a 4 percent rise in water vapor in the air column. In ground-based
records, about 76 percent of weather stations in the United States have
seen increases in extreme precipitation since 1948. One analysis found
that extreme downpours are happening 30 percent more often. Another
study found that the largest storms now produce 10 percent more
precipitation.

William Lau, a scientist at NASA’s Goddard Space Flight Center,
concluded in a 2012 paper that rainfall totals from tropical cyclones in
the North Atlantic have risen at a rate of 24 percent per decade since
1988. The increase in precipitation doesn’t just apply to rain. NOAA
scientists have examined 120 years of data and found that there were
twice as many extreme regional snowstorms between 1961 and 2010 as there
were from 1900 to 1960.

But measuring a storm’s maximum size, heaviest rains, or top winds
does not capture the full scope of its power. Kerry Emanuel, a hurricane
expert at the Massachusetts Institute of Technology, developed a method
to measure the total energy expended by tropical cyclones over their
lifetimes. In 2005, he showed that Atlantic hurricanes are about 60
percent more powerful than they were in the 1970s. Storms lasted longer
and their top wind speeds had increased by 25 percent. (Subsequent
research has shown that the intensification may be related to
differences between the temperature of the Atlantic and Pacific oceans.)

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